Are There Affordable Alternative Sputtering Targets to Gold?(Substitution strategies for low-vacuum coaters amid soaring gold prices)

Tingting Z

17. Juni 2025

International gold prices have climbed steadily over the past six months, sharply increasing the cost of gold (Au) targets that are routinely used in scanning electron-microscopy (SEM) sample coating. In low-vacuum sputter coaters such as the VPI SD-900 and SD-900M, gold targets are the traditional choice for rendering non-conductive specimens conductive and improving image quality. Under present market conditions, however, laboratories have strong incentives to identify lower-cost or higher-performance alternatives.

This report assesses the feasibility of replacing gold targets with silver (Ag) or platinum (Pt) targets in low-vacuum sputter coaters for both conventional SEM and field-emission SEM (FE-SEM). We compare coating performance, electrical conductivity, film quality, imaging resolution, instrument compatibility, long-term stability, and overall cost effectiveness, and provide a side-by-side data table to help users weigh the advantages and limitations of each material.

1 Material Property Comparison

Property	Gold Target (Au)	Silver Target (Ag)	Platinum Target (Pt)
Bulk resistivity (20 °C)	2.2 × 10⁻⁸ Ω·m	1.6 × 10⁻⁸ Ω·m	1.1 × 10⁻⁷ Ω·m
Conductivity (Cu = 100)	≈ 71 (high)	≈ 108 (highest)	≈ 16 (low)
Typical grain size of sputtered film	~ 5–15 nm	~ 5–20 nm	~ 2–7 nm
Relative deposition rate	1.0 × (baseline)	~ 1.0 ×	~ 0.4 ×
Typical film thickness required	~ 10 nm	~ 10 nm	~ 2–5 nm
Secondary-electron yield	High	Moderate-to-high	High
Chemical stability in air	Excellent (non-oxidising)	Poor (tarnishes; halogen-sensitive)	Excellent (non-oxidising)
Useful magnification limit	~ 50 000 ×	~ 30 000–50 000 ×	≥ 100 000 ×
EDS interference	Au M/L lines overlap P/S	Minimal; Ag lines avoid P/S	Pt M/L lines produce extra peaks

Key points

Silver offers the highest conductivity and costs less than gold, though its grains may appear slightly coarser and the film is susceptible to tarnishing.
Platinum produces the finest grains, enabling ultrahigh-magnification imaging with minimal surface texture, but has lower conductivity and a slower sputter rate (mitigated by thinner films).
Gold remains a balanced, “all-round” choice but has become the most expensive option and limits ultimate resolution due to grain visibility.

2 Film Quality and Reliability

1. Uniformity & Coverage

o Pt: Continuous coverage with only 2–5 nm thickness; ultrasmooth at ≥ 100 k× magnification.

o Au & Ag: Require ~ 10 nm to close surface gaps; Au grains (~ 5 nm) become visible above ~ 50 k×, Ag similar or slightly coarser and may enlarge on halogen-rich substrates.

2. Adhesion & Internal Stress

o Au and Ag films are soft and adhere well under typical low-vacuum conditions.

o Pt is harder and more prone to stress-induced micro-cracking if residual O₂/H₂O is present; higher vacuum or inert-gas purging is recommended.

3. Environmental Stability

o Au and Pt films are inert and retain conductivity indefinitely.

o Ag films tarnish in air containing S, Cl, or halogen vapours; long-term storage demands sealed, dry, low-sulphur environments or re-coating before imaging.

3 Imaging Performance

Charge Suppression & S/N – All three metals eliminate charging once a continuous film forms. Silver’s superior conductivity theoretically evacuates charge fastest; in practice, ≥ 2 nm of any metal suffices for most samples.
Resolution Limits – Above ~ 50 k×, Au and Ag grain texture becomes discernible, constraining resolution; Pt remains essentially grain-free up to ≥ 100 k×, making it ideal for nanostructures.
EDS Considerations – Au M lines overlap P and S Kα peaks; Pt M/L lines interfere with some light elements. Ag L lines (~ 3 keV) lie away from P/S, reducing spectral overlap; hence Ag is often preferred for biological or geological samples rich in P, S, or Cl.

4 Instrument Compatibility and Practical Use

Low-vacuum sputter coaters (e.g., VPI SD-900/SD-900M) accept conductive targets of identical dimensions, so Au, Ag, and Pt discs are interchangeable.

Ag runs at nearly identical voltage/current settings to Au; no major parameter changes.
Pt needs longer sputter times or slightly higher power owing to its slower rate, but the thinner film means total run time may be comparable.
Maintain good vacuum (≤ 10 Pa) and argon purging when coating with Pt to minimise oxygen-related film cracking.
Store Ag targets in desiccated, sealed containers; brief initial sputtering removes surface oxides before routine coating.

5 Application Guidelines

Target	Recommended Use-Cases	Remarks
Gold (Au)	General-purpose SEM coating; routine magnifications < 50 k×	Well-balanced performance but now the most expensive; interferes with P/S EDS.
Silver (Ag)	High-throughput, cost-sensitive labs; teaching; imaging ≤ 50 k×; samples requiring clear P/S EDS	Lowest cost; highest conductivity; film tarnishes—coat–observe promptly; avoid halogen-rich specimens.
Platinum (Pt)	FE-SEM; ultrahigh resolution (> 50 k×); long-term archival samples; critical nanostructure studies	Finest grain, chemically inert; slower deposition offset by thinner film; higher initial price but cheaper than gold at current spot prices.

Amid persistently high gold prices, silver targets provide an economical, high-conductivity substitute for routine SEM work, while platinum targets deliver unmatched high-resolution performance and chemical durability for demanding applications. Laboratories can flexibly adopt one or more targets to balance budget constraints against imaging requirements. As material science advances and market dynamics evolve, Ag and Pt are poised to play an ever greater role in SEM sample preparation, offering researchers practical, high-value alternatives to gold.